Curable silicone compositions are used as sealing materials or protective coating materials for optical semiconductor elements in optical semiconductor devices such as light emitting diodes (LEDs). Because cured products of curable silicone compositions typically exhibit high gas permeability, when such cured products are used in high brightness LEDs, which exhibit high light intensity and generate large amounts of heat, problems occur such as discoloration of the sealing material due to corrosive gases and a reduction in brightness due to corrosion of silver plated on the LED substrate.
In order to solve such problems, Japanese Unexamined Patent Application Publication No. 2012-052035A proposes a curable silicone composition comprising a diorganopolysiloxane which is capped at both molecular terminals with alkenyl groups, has a short main skeleton, has no adjacent diphenylsiloxane units, and has a methyl group in the center, an organohydrogenpolysiloxane, and a hydrosilylation reaction catalyst.
However, in the composition described above, it is difficult to ensure that no diphenylsiloxane units are adjacent to one another and to prepare a diorganopolysiloxane having a methyl group in the center. In addition, when the content of diphenylsiloxane units exceeds the content of other diorganosiloxane units, the viscosity of the curable silicone composition increases, which diminishes handleability, and there is also the problem that the mechanical characteristics of a cured product obtained by curing the composition are reduced.
An object of the present invention is to provide a curable silicone composition having excellent handleability and forming a cured product with a high refractive index and low gas permeability when cured. In addition, another object of the present invention is to provide a cured product having a high refractive index and a low gas permeability and to provide an optical semiconductor device having excellent reliability.